The present invention relates to the transmission of digital data and, more particularly, to high-definition television.
A problem arises in the context of the transmission of high-definition television (HDTV) signals in channels that are allocated for standard, so-called NTSC TV transmission. The problem specifically arises in areas where channels that are unused for NTSC transmission --and are thus candidates for HDTV transmission --are used for NTSC transmissions in relatively proximate areas. As an example, TV channel 3 is currently unused in the New York City metropolitan area, but is used in both Philadelphia and Hartford. The consequence of such proximity is that if channel 3 is to be used for HDTV in New York City, the HDTV signal may be corrupted by the channel 3 NTSC signal from Philadelphia or Hartford for New York City HDTV viewers. Conversely, the NTSC signal may be corrupted by the HDTV signal. Such corruption is referred to as "co-channel interference."
The co-channel interference from the HDTV signal to the NTSC signal can be satisfactorily dealt with by specifying in the HDTV standards (still under consideration) a sufficiently low transmit power level. Moreover, in order to deal with co-channel interference from the NTSC signal to the HDTV signal, it has been proposed to rely on the fact that an NTSC signal is dominated by energy concentrations at particular locations in the frequency spectrum. Thus, it has been proposed to provide an HDTV receiver with a filter, hereinafter referred to as an NTSC rejection filter, having nulls at those frequency locations, thereby removing a significant portion of the interfering NTSC signal, while only minimally degrading the HDTV signal.
The proposed HDTV standards envision a digital transmission format in which the bit stream representing the baseband HDTV signal is mapped, via a so-called channel code, into channel symbols each comprised of one or more so-called signal points. A potential problem in such arrangements is that the NTSC rejection filter creates so-called forced intersymbol interference (ISI) in the receiver, which corrupts the received signal points.